Low-color density color materials, mainly such as blue color materials, are being used as color materials that can be decolorized using a decolorizing apparatus.
When a document printed with such a decolorable color material of a specific color is read and copied, the density of the scanned and read image (input density) is lower than that when a general black color material is used. When the read image is printed on a sheet, the density of toner on the sheet (output density) becomes low because the color is reproduced with the color of the used color material. Therefore, the reproduced color is further lighter, so that the image printed on the sheet is hard to read.
To legibly print, using a conventional monochrome image forming apparatus, a document printed with a color material of a specific color, it is necessary to adjust image quality by the user through a touch panel or to adjust the density of the image using self-diagnostic adjustment set by a serviceman. However, in this case, to obtain the print results desired by the user, it is necessary to repeat printing several times to check the results.
In addition, when a color document is scanned in monochrome such as grayscale, the color image is converted to a monochrome image. In the converted monochrome image, the color of the color material printed on the scanned original sheet (for example, blue) cannot be distinguished from the colors of notes and stains on the sheet (for example, black and brown). Therefore, when the output density is set to be high to make the print results legible, the notes and stains are also emphasized.